1. Field of the Invention
Embodiments of the present invention relate to an apparatus that measures a thickness of a coating applied on a surface of an object, and more particularly, to an apparatus that measures a thickness of a coating by selecting a wavelength of a laser based on a color of the coating using a contactless method using a photoacoustic effect and an interference signal, rather than an existing contact method.
When a pulsed laser having a nanosecond pulse width is applied to a surface of the coating, an ultrasonic wave may be generated on the surface of the coating and thus, the surface of the coating may be displaced and an optical interference signal may be changed. By measuring the changed optical interference signal, the thickness of the coating may be measured.
To achieve an efficient photoacoustic effect, a pulsed laser having a wavelength band that is excellently absorbed into the coating may be used. To measure an optical interference signal with an excellent sensitivity, a continuous wave (CW) laser having a wavelength band that is excellently reflected by the coating may be used. Since an absorptivity and a reflectivity of the coating vary depending on a color of the coating, lasers having different wavelengths may be used based on the color of the coating. Provided is a wavelength-selectable coating thickness measurement apparatus that may include CW lasers and pulsed lasers having a number of wavelength bands, and select a laser having a desired wavelength band. In another example, provided is a wavelength-selectable coating thickness measurement apparatus that may configure a sensor head unit including a single pulsed laser and a single CW laser, and replace the sensor head unit based on a color of the coating.
2. Description of the Related Art
Coating thickness measurement apparatuses currently being used for vessels, aircrafts, and automobiles may be classified based on measurement principles into a radiographic coating thickness measurement apparatus, an ultrasonic coating thickness measurement apparatus, an eddy current coating thickness measurement apparatus, and a magnetic induction coating thickness measurement apparatus. A method using radiation may detect a defect in transmissive and reflective manners using an X-ray or a gamma ray. The method using radiation may have an advantage of contactless measurement and a disadvantage of harmfulness of radiation to a user. A method using an ultrasonic wave may measure a thickness by measuring a travelling time of an ultrasonic wave transmitted to a sample, reflected, and returned. The method using an ultrasonic wave may require a medium to be used for impedance matching between an ultrasonic transducer and the sample and thus, an application of the method using an ultrasonic wave to a contactless measurement method may be difficult. An eddy current method may measure a thickness based on a change in resistance of an electrode caused by eddy current. A magnetic induction method may measure a thickness by measuring an increase in voltage and an increase in magnetic flux. The magnetic induction method may have an advantage of portability and a disadvantage of contact measurement. Since such a contact measurement apparatus may perform measurement after a covering is completely dried, a great deal of time and expenses may be used to provide dry air for a long time.
In a case of contactless covering thickness measurement using light, the measurement may be performed in real time during a continuous process. Thus, the period of ship constructions may be reduced, and the measurement may be performed rapidly and precisely based on characteristics of light.
An absorption spectrum and a reflection spectrum of a laser may vary depending on a color of a coating. Thus, different wavelengths may be selected for a pulsed laser and a CW laser based on the color of the coating. When a color of a coating to be measured is determined, a pulsed laser having a high-absorptivity wavelength band may be selected to produce an excellent photoacoustic effect, and a CW laser, to be used for an optical interferometer, having a high-reflectivity wavelength band may be selected to be excellently reflected on a surface of the coating.
By integrally including pulsed lasers and CW lasers having different wavelengths in a single measurement apparatus, or by housing pulsed lasers and CW lasers having different wavelengths in an individual module and replacing the module based on a color of a coating, thicknesses of coatings in many colors may be measured using a single measurement apparatus.